Silver-containing direct thermographic imaging materials are non-photosensitive materials that are used in a recording process wherein images are generated by the direct application of thermal energy and in the absence of a processing solvent. These materials have been known in the art for many years and generally comprise a support having disposed thereon one or more imaging layers comprising (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing agent composition (acting as a black-and-white silver developer) for the reducible silver ions, and (c) a suitable binder. Thermographic materials are sometimes called “direct thermal” materials in the art because they are directly imaged by a source of thermal energy without any transfer of the image or image-forming materials to another element (such as in thermal dye transfer).
In a typical thermographic construction, the image-forming thermographic layers comprise silver salts of long chain fatty acids. The preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, such as behenic acid or mixtures of acids of similar molecular weight. At elevated temperatures, the silver(I) of the silver carboxylate is reduced, by a black-and-white reducing agent (also known as a developer) whereby elemental silver is formed. Imagewise heating, such as by using a thermal print-head, results in a black-and-white image.
Problem to be Solved
As noted above, direct thermographic materials are imaged by a recording process whereby images are generated by imagewise heating a recording material containing chemical components that change color or optical density in an imagewise fashion. Such materials generally include one or more thermographic (imaging) layers on a polymeric support. Such thermographic layers generally include the imaging chemistry dispersed in one or more polymer binders. For example, U.S. Pat. No. 6,436,622 (Geisler) describes direct thermographic materials that include an imaging layer containing polyvinyl acetals (such as polyvinyl butyral and polyvinyl formal) as the binder. Various other polymeric materials are also described as potential binders
Japanese Kokai 2004-279499 (Onuma et al.) and 2004-279500 (Onuma et al.) describe the use of polyvinyl acetal resins and resin mixtures of various molecular weights in photothermographic materials.
We have found however, that not every hydrophobic binder mentioned in the literature provides the same advantages when imaging direct thermographic materials. For example, we have found that the maximum achievable image density may be reduced when a polyvinyl acetal having a low degree of polymerization is used as the binder in the thermographic layer. One skilled in the art might recover that loss in image density by adding more organic silver salt but this obviously increases the cost of the material and is not an efficient use of silver.
Attempts to achieve a higher image density through either longer contact time with the print-head or by increasing the print-head temperature are often unsuccessful and actually result in a “fall-off” in the maximum image density that can be achieved.
Thus, there is a need to find hydrophobic binders for use in thermographic layers that achieve improved silver efficiency by providing high image density and at the same time use less silver. Such hydrophobic binders also should be capable of providing higher image densities before the initiation of “fall-off.”